Photosensitive materials so far used in electrophotographic photosensitive materials (hereinafter also referred to as photosensitive materials) include inorganic photoconductive substances, such as selenium and selenium alloys, dispersions of inorganic photoconductive substances, such as zinc oxide and cadmium sulfide, in resin binders, organic photoconductive substances, such as poly-N-vinylcarbazole and polyvinyl-anthracene, organic photoconductive substances, such as phthalocyanine compounds and bisazo compounds, and dispersions of such organic photoconductive substances in resin binders.
Photosensitive materials are required to have the function of holding surface charges in the dark, the function of receiving light and generating charges and the function of receiving light and transporting charges. There are two kinds of photosensitive materials, namely the so-called monolayer type photosensitive material consisting of one single layer having all the three functions and the so-called laminate type photosensitive material composed of functionally distinguishable layers, namely a layer which contributes mainly to charge generation and a layer which contributes mainly to retention of surface charges in the dark and charge transport upon receiving light. In electrophotographic image formation using these photosensitive materials, the technique of Carlson, for example, is applied. Image formation by this technique includes charging of the photosensitive material by corona discharge in the dark, formation of latent electrostatic images (e.g. letters, pictures) by illumination of the charged photosensitive material surface, development of the latent electrostatic images thus formed with a toner and fixation of the developed toner images on a supporting material, such as a paper sheet, following transfer thereto. After toner image transfer, the photosensitive material is subjected to the steps of charge removal, removal of remaining toner (cleaning), neutralization of residual charge by means of light (erasure), and so on, and then submitted to reuse.
In recent years, electrophotographic photosensitive materials in which organic materials are used have been put to practical use because of their advantageous features such as flexibility, thermal stability and film forming property. Thus, for example, there may be mentioned photosensitive materials comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluorene-9-one (described in U.S. Pat. No. 3,484,237), photosensitive materials in which an organic pigment is used as the main component (described in Japanese Patent Application (OPI) No. 37543/1972) (the term "OPI" as used herein means "unexamined published Japanese Patent Application") and photosensitive materials in which a eutectic complex is used as the main component (Japanese Patent Application (OPI) No. 10735/1972). A number of novel hydrazone compounds have also been put to practical use.
However, although organic materials have a number of advantageous featues as compared with inorganic materials, none of organic materials can fully meet all requirements set forth with respect to the characteristic properties of photosensitive materials for electrophotography. Organic materials are still unsatisfactory particularly in respect of photosensitivity and of characteristics at the time of continuous repeated use.